1. Field of Invention
The invention disclosed in this document is an illuminator that outputs linearly polarized light. More particularly, the illuminator is suitable for incorporation in LCoS microdisplay based light engines of the type used in video projector applications.
2. Discussion of Background
A block drawing of a set of components behind the screen of a LCoS microdisplay based rear projection video display (such as a TV) is illustrated in FIG. 1. To the left of the figure, the light source and condenser elements are collectively called the illuminator.
The light source is usually a mercury short arc lamp. This type of lamp emits an output light beam that is round and has a spatially non-homogeneous intensity profile. One function of the condenser is to shape and size the output light beam to match that of the rectangular microdisplays of the kernel and to homogenize the light intensity within the output light beam. Other functions of the condenser are to set the f# of the output light beam and to remove the infrared and the ultra violet portions of the light source emission spectrum. Some condensers also remove undesired portions of the visible spectra from the light beam. (A common example of this function is the removal of the “yellow” spike from the emission spectra produced by the mercury short arc lamp.) Additional requirements placed on the illuminator are that it be physically compact and inexpensive.
The subject of this disclosure relates to the polarization of the light output by the illuminator. The lamp itself outputs unpolarized light. In many applications the illuminator also outputs unpolarized light and this is acceptable. Other applications require that the illuminator output linearly polarized light.
There are a number of conventional techniques known to the art by which to produce an illuminator that outputs linearly polarized light. Three of the more widely used techniques are as follows:                The inclusion of a linear polarizer in the illuminator. The polarizer is typically a PBS or a reflective polarizer. In either case, the big disadvantage of this approach is the loss of a minimum of half the light produced by the illuminator.        The inclusion of a so-called “fly's eye” recombination system into the illuminator. Such an illuminator includes a lamp with a parabolic reflector, a fly's eye lens, a polarization rotator array plate and a second fly's eye lens. This type of system requires careful alignment and is quite expensive.        The inclusion of an integrating rod based recombination system into the illuminator. Such an illuminator includes a lamp with an elliptical reflector and a modified integrating rod. The modification consists of the addition of a reflective aperture and a quarter waveplate at the input end of the rod and the addition of a reflective polarizer at the rod output. The resulting illuminator certainly outputs linearly polarized light but the intensity is found to be only a modest increase over the approach of simply inserting a polarizer into the illuminator.        